A fieldbus is a network used in process automation in which field devices form some of the nodes on the network and a head station or host forms one of the nodes on the network. The network may include a trunk or home run connected to a power supply and spurs that extend from the trunk, with field devices forming nodes on the spurs. The function of the fieldbus network is to transmit data to and from the nodes in a reliable and timely manner.
A fieldbus network typically bases its communications protocols upon the Open Systems Interconnection model (OSI) that is maintained by the International Organization for Standardization (ISO) as ISO/IEC 7498-1. The OSI model defines a hierarchical, layered communications stack in which data is converted to and from data frames for transmission on the network. Each layer provides services to the layers directly above and below it.
The lowest layer of the OSI model is the physical layer. The physical layer handles converting data frames to and from the electrical signals transmitted on the network. The physical layer defines the physical and electrical specifications of the network (network topology, wiring specifications, voltages, line impedance, etc.) and how the data frame is represented on the network (for example, Manchester coding, return to zero coding, nonreturnable to zero inverted coding, and the like).
Many fieldbus networks utilize a two-wire loop defined by the physical layer specifications. The two-wire loop transmits power to the nodes and is used for data communications between the nodes and the host. Power is usually transmitted by a DC voltage carried on the loop, and data is communicated by superimposing an AC data signal on the DC voltage. Fieldbus networks that utilize a two-wire loop include FOUNDATION FIELDBUS H1, PROFIBUS PA, and ETHERNET based networks including POWER OVER ETHERNET (POE) networks (note that a wired fieldbus network may include other wires in addition to those used in the two-wire loop).
FIG. 1 schematically illustrates a fieldbus communications stack 10 having a physical layer 12 that is connected to a two-wire loop (represented by the single line 14) to transmit and receive data frames 16 over the loop 14. Each data frame 16 is defined by an AC data signal superimposed over a DC voltage 18. The data frames 16 are separated by a “quiet time” 20 between frames that avoids collisions and interference between data frames. The physical layer specifications define the encoding of the AC data signal and how collisions of data frames are avoided.
FIG. 2 schematically illustrates the encoding of a data frame 16 for a FOUNDATION FIELDBUS H1 fieldbus network (the DC component is omitted). The data bits are transmitted at a frequency of 31.25 kHz as indicated by the clock signal 22, that is, the bit time is 1/31,250th of a second.
The data portion of each data frame 16 is encoded using a synchronous Manchester coding technique in which a positive voltage change in the middle of the bit time represents a logical “0” and a negative voltage change in the middle of the bit time represents a logical “1”. Special codes are defined for the preamble 24, and for the start delimiter 26 and end delimiter 28 of the data frame 16.
A node on the fieldbus network uses the preamble 24 to synchronize its internal clock with the incoming data frame 16, and uses the start delimiter 26 to find the beginning of the data portion of the data frame 16. After finding the start delimiter 26, the node accepts data until receipt of the end delimiter 28. The start and end delimiters 26, 28 include N+ and N− signals that do not change voltage in the middle of the bit time to assist the node in recognizing the start and end delimiters.
FIG. 2 illustrates data 30 as including a sequence of logical bytes 10011010 which are encoded in the date frame 16 as data signal 32. The fieldbus communication protocol defines the maximum and minimum number of data bits that are included within a data frame 16.
Data signal 32 represents the ideal AC voltage signal that should be transmitted on the two-wire loop 14. Due to the physical characteristics of the two-wire loop 14 and the surrounding physical environment, the actual AC voltage signal will vary from the ideal voltage signal 32. FIG. 3 illustrates the actual AC voltage signal 34 as compared to the idealized voltage signal 32.
Some of the signal departures from ideal include signal noise, jitter, and overshooting. FIG. 4 illustrates signal noise having a maximum amplitude 36. FIG. 5 illustrates jitter 38, which is the time difference between the AC signal transition crossing the zero voltage line and the midpoint of the bit cycle. Fieldbus protocols typically define acceptable jitter limits 40. FIG. 6 illustrates a transient overshoot having a magnitude 42, as well as the AC signal having a DC offset having a magnitude 44. Other signal departures from the ideal include misshapen waveforms, too high or too low DC power levels, voltage spikes, and the like. Because these signal departures and waveform variations from the ideal are well known in the fieldbus art, they will not be discussed in further detail.
The demands of a standardized fieldbus communications protocol makes it difficult for a fieldbus network to communicate with other fieldbus networks having a different communications protocol.
Furthermore, the demands of a fieldbus communications protocol requires the physical layer of the fieldbus network be maintained for reliable operation of the network. Physical layer diagnostic devices are known that connect to the two-wire loop 14 and monitor the physical layer of the fieldbus network, measuring and evaluating bus voltage, signal noise, retransmission counts, shield shorts, signal levels, and other electrical and physical parameters and events as is known in the fieldbus physical layer diagnostics art.
An example known fieldbus physical layer diagnostic device is the Field Diagnostic Module sold by Phoenix Contact GmbH, Blomberg, Germany. The diagnostics module provides data from AC and DC electrical-related measurements useful for evaluating the health of the physical layer of the fieldbus network.
Some physical layer diagnostic devices are designed to be attached on the trunk near the power supply. However, especially in the case of isolated or voltage-regulated spurs, the electrical parameters on the spurs are not the same as the trunk. Additional information, such as current draw, of individual field devices may not be available to diagnostic devices attached to the trunk. Other diagnostic devices are designed to be attached to spurs and the diagnostic device includes communications circuitry to communicate over the field bus network. But adding such diagnostic devices to spurs is expensive.
Some fieldbus physical layer diagnostic devices incorporate an oscilloscope within the diagnostic device. The oscilloscope may be the sole diagnostic tool provided by the device, or may part of a set of diagnostic tools provided by the device. The oscilloscope enables a user to continuously monitor the fieldbus communications and view the signal waveforms. The oscilloscope provides diagnostic information about the waveforms for analysis.
Because oscilloscopes sample the waveform at a relatively high frequency (higher than the bit frequency of the waveform), delivering the data stream generated by the oscilloscope through the two-wire loop is impractical. Instead, the oscilloscope data stream is transferred out of the field device using a dedicated, higher bandwidth communication channel and not through the fieldbus network itself. The electrical connections between the fieldbus network and the oscilloscope communication channel must also be isolated from one another. This hinders use of an oscilloscope as an effective diagnostic tool for fieldbus networks.